In U.S. Pat. No. 4,598,763, a fluid-permeable casting ring was invested in the peripheral wall of the mold cavity, and was used to envelope the body of molten metal in an oil-encompassed annulus or "sleeve" of gas that assisted the body in escaping from the mold without galling. The patented sleeve-forming process is now widely used in casting aluminum, in particular, because of the superior surface qualities it provides, and the process is often supplemented by the gas-infused direct chill process disclosed in U.S. Pat. No. 4,693,298, which enhances the effects obtained by the sleeve-forming process.
To employ the sleeve-forming process, however, the casting ring must be invested so fluid-tightly in the peripheral wall of the cavity, that the fluid delivery system for the ring can introduce the gas and oil to the body of the ring without risk that the fluids will intermix with one another other than in the body of the ring as intended, and without risk that they will escape from the mold other than at the inner peripheral surface or "casting surface" of the ring, where the annulus or "sleeve" is formed. In short, the ring must be incorporated in the mold in such fluid-tight relationship with the same that the fluids can be introduced to the ring at certain strategic locations thereon which will assure that the fluids function as intended, yet the fluid-tight relationship between the ring and the mold will prevent the fluids from escaping other than at the casting surface of the ring.
A common technique for this purpose has been to machine the ring to the desired diameter on a lathe, and then while the ring is seated on a support in the peripheral wall of the cavity, to heat-shrink the mold around the ring in an oven, to produce an essentially fluid-tight relationship between the ring and the wall of the cavity at the outer peripheral face of the ring, due to the hoop-tension generated in the mold at the face of the ring. Given this "shrink-fit" around the ring, the fluids can be introduced then at separate points on the outer peripheral face of the ring and one axial end thereof, and can be expected to intermix with one another only as intended in the body of the ring, and to escape only at the casting surface of the ring.
The technique is limited, however, to the formation of a cavity having a cylindrical cross-sectional outline at the peripheral wall thereof. It cannot be used to confer an angulated cross-sectional outline on the cavity, and even when the technique is used to confer a cylindrical cross-sectional outline on the cavity, the larger cylindrical sizes often call for raw material sizes, oven sizes, and machining techniques which are outside of the standards of everyday practice used in the moldmaking industry.